LE & MLE Processes

Ludzack-Ettinger (LE) and Modified Ludzack-Ettinger (MLE) Processes

In 1962, Ludzack and Ettinger proposed the addition of an an anoxic zone at the head of a conventional aerobic plant to utilize the readily biodegradable substrate (RBCOD) as a denitrification source. In the early 1970’s, James Barnard suggested the addition of a mixed liquor recycle to return nitrate back to the anoxic zone, and named the process Modified Ludzack Ettinger. The MLE is regarded as the first successful nitrogen removal activated sludge process.

The MLE process (unlike earlier attempts such as LE and Wuhrman), achieved robust and efficient removal by satisfying all of the requirements of nitrification and denitrification. The anoxic zone requires conditions which are free of dissolved oxygen, but needs organic carbon compounds to provide energy (electron donors), the presence of nitrate (electron acceptors) and a facultative biomass (bacteria capable of denitrification) for efficient denitrification.

Operating sludge ages should be sufficiently high to facilitate nitrification, and the aerobic basin should be sized to ensure that sufficient aerobic retention time is available to accommodate operation at low temperatures.

The process operates efficiently at TKN/COD ratios of >0.1and the ultimate efficiency depends upon a number of factors, including the flow rate of anoxic mixed liquor recycle (‘a’-recycle) – which might vary between 2 and 6 times the influent flow rate to achieve optimum denitrification.

The denitrifation process is adversely affected by high levels of DO in the ‘a’-recycle which may limit the ‘a’-recycle flow and the denitrification capacity. The process will not denitrify completely.

Efficient denitrification is also dependent upon the size of the anoxic zone and the relative amounts of soluble and particulate biodegradable organic carbon in the influent. Anoxic retention time should be sufficient to permit the denitrification capacity of the anoxic zone to be just met, and may vary significantly depending on the above considerations but is usually 1-3 hours.

The denitrification process returns about 4.57 mg of alkalinity for each mg of nitrate-N denitrified and as a consequence provides a more stable pH environment than the fully aerobic process. Many of the modern process developments are based upon this process including the Bardenpho, Phoredox, Johannesburg, UCT and Modified UCT.

In recent years, a number of aerobic plants have incorporated a small anoxic zone as an anoxic selector. The role of the anoxic selector is to encourage growth of floc-formers in a high F/M environment, and minimize filament growth by eliminating RBCOD entering the aerobic zone. This upgrade is often called an LE plant upgrade, and optimization can be investigated in SASSPro by setting the ‘a’ recycle to zero.

This process is efficient and robust since it provides all of the requirements needed for denitrification, and is controlled through variation of the mixed liquor recycle (‘a’-recycle).

Denitrification process efficiency is affected by the size of the anoxic basin, the TKN/COD ratio, the ‘a’-recycle rate and the concentration of dissolved oxygen in the ‘a’-recycle. High levels of DO in the ‘a’-recycle limit both the ‘a’- recycle flow range and the denitrification capacity.

Typically, ‘a’-recycle rates of 2-6 Qin are usually required to provide optimum nitrogen removal. The MLE process will not denitrify completely.

The MLE process is most efficient with typical Influent TKN/COD ratios of > 0.10.

In recent years, a number of aerobic plants have incorporated a small anoxic zone as an anoxic selector. The role of the anoxic selector is to encourage growth of floc-formers in a high F/M environment, and minimize filament growth by eliminating RBCOD entering the aerobic zone.

This upgrade is often called an LE plant upgrade, and optimization can be investigated in SASSPro by setting the ‘a’ recycle to zero and using the individual reactor status reports to optimize the process configuration. Parameters impacting on optimum process operation will be shown in red.